Method and system for the transmission of data and power

ABSTRACT

Power and data are both transmitted over a single transmission line to which a power source and a number of stations are connected. A bipolar AC voltage is supplied to the line by the power source, the half cycle pulses of one polarity being used for supply of power only, while transmission of data is achieved by modulation exclusively of the half cycle pulses of the other polarity. High power is supplied to the stations by providing pulses of the one polarity at a low ohmic level. Only low power is required for modulation for the purpose of data transmission, and so the pulses of the other polarity are supplied at a relatively high ohmic level. Thus, only two conductors are required, one of which can be replaced by the system ground in the simplest case. If three conductors are provided, it is possible to achieve improved tolerance of defects in a conductor. At least parts of the system are maintained in operation, in the event of a short-circuit, by providing separation units which automatically disconnect different segments of a short-circuited line and automatically reconnect them after the short-circuit has been eliminated.

BACKGROUND OF THE INVENTION

The invention relates to the transmission of data and power via a commontransmission line to which a power source and several stations areconnected, each of the stations essentially being capable of receivingpower and data via the transmission line and of transmitting data viathe transmission line.

If one and the same line is used for the transmission of power and ofdata, this line need comprise only few individual conductors or wires,no more than two conductors in the extreme case. This considerablyreduces the cost of the line, especially in systems having a widespatial distribution. This principle has accordingly been used invarious forms in the past.

DE 39 07 652 A1 discloses a circuit arrangement in which a currentsupply unit delivers a DC voltage to a two-wire bus line to whichseveral stations are connected. An energy storage device in the form ofa capacitor is present in each individual station, is charged by the DCvoltage on the bus lines via a rectifier and a resistor, and serves asthe current supply to the electronic circuit of the station. For datatransmission, the bus lines are intermittently short-circuited independence on the data to be transmitted. This means, however, that thecurrent supply unit must deliver the DC voltage to the bus line at acomparatively high-ohmic level, so that only a limited number ofstations can be supplied through this line, and in particular no highloads can be supplied at the stations.

DE 41 38 065 A1 discloses a device for the transmission of data andpower via a transmission line in which said line comprises two wires anda common screen inside which the two wires are accommodated. Theindividual stations are connected to the two wires and to the screen,receiving half the operational current through each wire, while the fulloperational current is returned through the screen. The data aretransmitted on both lines by means of voltage signals in counterphase.The power is derived from the two conductors by means of current sinksin the stations so as to load said voltage signals as little as possiblewith the power transmission. This requires comparatively expensive meansfor achieving a substantially constant DC voltage for supplying thecircuits in the stations, and the power which can be transmitted to thestations is limited also in this case.

It is an object of the invention to provide a method as well as a systemfor the transmission of data and power via the same line in which thepower which can be transmitted is essentially not limited, whilenevertheless data can be supplied at a high-ohmic level in a veryreliable transmission mode of the line.

SUMMARY OF THE INVENTION

According to the invention, this object is achieved in that an ACvoltage is transmitted through two transmission lines, the voltage ofone polarity being used for electrical power supply of the stations andthe voltage of the other polarity being used for data transmission. Thetwo conductors are thus used in time multiplex for the transmission ofelectrical power and the data, the stations being supplied from a localenergy storage device during data transmission, which device is chargedagain during the power transmission phase. The polarity reversal rendersit possible to arrange the electrical supply and the data transmissionwith the highest degree of independence from one another. Thetransmission method for data and the static and dynamic properties ofthe electrical supply, and also of the application loads in thestations, as applicable, are decoupled from one another both by the timesequence and also by the polarity change. Reliable data transmission canthus also be achieved with the simplest type of modulation.

It is important for reliable operation of such a system that thisdecoupling is already provided by the use of the AC voltage, and thustakes place without active synchronization of the stations themselvesand without actions taking place in the stations, such as, for example,an operational switch-over. This decoupling at the stations can besimply realized by means of passive components if the current is derivedfrom the conductors in only one direction for the electrical supply, forexample by means of diodes, while the current is modulated in the otherdirection for the transmission of data only. The avoidance of an activemode switch-over or synchronization in the stations leads to a verysimple and reliable system.

The power source, in the form of a generator which introduces the ACvoltage into the two conductors, can generate this voltage without anyrelation to ground potential. Therefore a short-circuit of a conductorwith the system ground or with some other supply line is acceptable andrequires no corrective actions whatsoever.

Given a suitable arrangement of the system, moreover, conductorinterruption defects can also be accommodated if a third conductor isused as a replacement for the interrupted conductor. The system groundmay also be used as the replacement for the interrupted conductor, inwhich case the system function is retained in such a back-up mode, butthe differential operation typically will no longer effectivelycontribute to the electromagnetic compatibility (EMC) of the system.When the system ground is used as the replacement conductor, ashort-circuit of a conductor can no longer be accepted at the same time,depending on the arrangement.

To protect the generator supplying power to the lines in the case of ashort-circuit, the defective conductor, or one of the defectiveconductors may be disconnected by means of a fuse in the generator. Thismeans, however, that power supply and data transmission will not bepossible to the connected stations in such embodiments of the systemaccording to the invention. It is possible to insert at least oneseparation unit in at least one of the conductors so as to be able tooperate at least some stations in the case of a short-circuit, whichseparation unit automatically interrupts the relevant conductor in caseof a short-circuit. The system can thus be operational with at leastsome of the stations in that case. Especially if the conductor system isconstructed as a ring line and several distributed separation units areused, it is possible for major portions of the system to remainoperational in case of short-circuits.

The separation units each comprise at least one monitoring unit and oneswitch, which monitoring unit controls the switch in the relevant linesuch that the switch is opened in the case of short-circuits oroverloads. The switch is shunted by a current-limiting element. Thepotential is thus transferred to the conductor portion following theseparation unit at a high-ohmic level. The monitoring unit controls theswitch in the separation unit on the basis of this potential. When ashort-circuit or an overload has been eliminated, the switch willautomatically be closed by the monitoring unit.

The switch preferably comprises two field effect power transistors(FETb) of the same conductivity type whose sources are interconnected.Their gates are also interconnected and coupled to a control transistorof the monitoring unit. The control transistor is preferably also afield effect transistor, of opposite conductivity type to the powertransistors in the switch, and is connected to the second conductorwhich during normal operation has a different potential from that of theconductor in which the switch with the power transistors is included.The monitoring unit monitors the potential of the connected conductorportions. If this potential rises above a first triggering threshold,which lies above the threshold voltage of the field effect transistorsof the monitoring units, the control transistor becomes conducting andconnects the non-interrupted conductor to the gates of the powertransistors in the switch. This renders the power transistors in theswitch conducting, so that the switch is closed. In case ofshort-circuits or overloads, the potential at the connected conductorportions drops to below a second triggering threshold which may be equalto the first triggering threshold, so that the control transistors ofthe monitoring unit, and accordingly the power transistors of theswitches to be controlled become nonconducting, which interrupts theconductor and effectively separates the short-circuited conductorportion from the remaining operational part of the conductor system.

Since the control transistor is capable of driving the power transistorsof the switch into conduction in one half wave of the AC voltage only,the switch is supplemented with a power storage element. This powerstorage element preferably is a capacitor which is charged, for example,during the positive half wave and stores up the necessary power forkeeping the switch closed also during the negative half wave. Thiscapacitor is connected between the junction point of the two sources ofthe power transistors of the switch and the junction point of theinterconnected gates of the power transistors. Short-circuits occurringonly during that half wave in which the switch is closed by the storedcharge of the capacitor, however, are not recognized. It is achieved bya further provision, however, that the separation unit also recognizesshort-circuits during the second half wave.

The conductivity type of the power transistors in the switch determinesin which half wave short-circuits are recognized.

The current-limiting element which shunts the switch is preferably ahigh-ohmic resistor or a controllable resistor combination, and isdenoted as test resistor below. The potential is transferred to theoutput of the separation unit via this test resistor when the switch isopened. When the short-circuit is eliminated, a potential necessary fortriggering the monitoring unit arises across the test resistor, and theswitch is closed automatically.

The test resistor may be advantageously operated in steps. For thispurpose, the current-limiting element comprises several ohmic resistorsconnected in series, individual resistors being shunted by switchingtransistors. High-ohmic and low-ohmic test resistances can thus berealized. First the potential is conducted from the input of theseparation unit to the output of the separation unit via a high-ohmictest resistor. If the monitoring unit now detects a short-circuit, theswitch in the separation unit remains open, i.e. the transistors in theswitch are non-conducting. The test resistor is switched over to alower-ohmic value only when a potential is detected which lies above thefirst triggering threshold, while the switch is closed.

An increase in the detection reliability of short-circuits is achievedby means of a hysteresis for the triggering threshold of the controltransistor of the monitoring unit. During normal operation with closedswitch, the triggering threshold is higher so as to register a potentialdrop quickly in the case of a short-circuit and to open the switchquickly. The triggering threshold is lower when the switches are open,with the object of achieving a faultless test for short-circuits via thehigh-ohmic test resistor.

It is ensured through the use of such separation units that ashort-circuit, for example in the supply line to a station, will nothamper a communication between other stations.

It is an advantage of the invention that the failure tolerance operateswithout actions, for example mode switch-over, whereby a highreliability and consistency of the system is achieved. A furtheradvantage is that the differential transmission remains intact also inthe case of defects, and thus the advantages of the low radiation level,a high degree of immunity to incoming interference, and a high toleranceof ground level shifts between stations are retained. A furtheradvantage is that the time reference for the implementation of the datatransmission protocol may be very unsharp, or may be entirely absent,because the starting point of the AC voltage already provides a reliabledemarcation in time from which a reliable scanning of the bits in a datatelegram is derived.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the invention will be explained in more detail below withreference to the drawing, in which:

FIG. 1 shows the construction principle of a system with a power sourceand several stations which are interconnected by means of twotransmission lines,

FIG. 2 shows a first embodiment of the construction of a station,

FIG. 3 shows an example of the voltage gradient on the two lines withdata modulation,

FIG. 4 shows a first embodiment of the construction principle of thepower source,

FIG. 5 shows the construction of a power source with a transformer,

FIG. 6 shows the construction of a power source with a transformerhaving a central tap at the secondary side for a three-line system,

FIG. 7 shows a further embodiment of the construction of a station whichis particularly suitable for the three-line system,

FIG. 8 shows the construction of a power source for a three-line systemin which the third line is used in the case of defects only,

FIG. 9 shows the construction of a system with three lines, where allthree lines are independent of a ground connection,

FIG. 10 shows the construction of a system with only a single line, inwhich the ground connection is used as the return line,

FIG. 11 is a block diagram showing the use of separation units,

FIG. 12 is a block diagram of the separation unit in the line having thehigher potential,

FIG. 13 is a detailed circuit diagram of a separation unit,

FIG. 14 is a detailed circuit diagram of a separation unit withtransistors of opposed conductivity types,

FIG. 15 is a circuit diagram of a separation unit, supplemented withelements for recognizing short-circuits in both half waves, and

FIG. 16 is a circuit diagram of a separation unit supplemented withelements for generating a hysteresis for the triggering threshold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a system for the transmission of data in which an ACvoltage is generated by a generator 10 and fed into two lines 11, 12.The connected stations 13, 14, 15 are electrically supplied by thepositive half wave of the AC voltage and transmit data during thenegative half wave of the AC voltage.

FIG. 2 shows the construction of one of the stations, for example thestation 14, where a capacitor 23 is charged via a diode 21 during thepositive half wave, from which capacitor the various components of thestation are electrically supplied by means of a voltage controller 25,for example a circuit 26 for carrying out the data transmissionprotocol, a dedicated control circuit 27, and a dedicated load 28. Thenegative half wave may be simply amplitude-modulated by means of aswitch 24 and a further diode 22 under the control of thedatatransmission protocol circuit 26. The amplitude of the negative halfwave is evaluated by a comparator 29 whose differential thresholdvoltage is negative, see also FIG. 3.

FIG. 3 shows the principle of the voltage gradient in time. The positivehalf wave 31 serves for the electrical supply to the stations, thenegative half wave 32 may have its amplitude modified by means ofswitches 24 in the stations. The comparator 29 in each station has anegative differential reception threshold 34 so that the amplitudemodulation can be evaluated in the simple manner. For example, a phaseof the negative half wave may represent a bit, the full amplitude--halfwave 32--denoting the rest state or idle state, and the reducedamplitude--half wave 33--the active state. The active state is achievedthrough closing of the switch 24 in one or several stations, such that asimultaneous transmission from several stations takes place withoutinterference, and all CSMA (Carrier Sense Multiple Access) protocols canbe used in principle, as well as their Data Link Layer such as, forexample, Ethernet, CAN, and J1850. The active state may then represent alogic bit 0 and the rest state a logic bit 1, or vice versa.

Since the individual bits are clearly demarcated in the stations by therelevant interposed positive half waves, simple protocols can be usedwhich deal satisfactorily with a complete telegram without anymechanisms for synchronization and for a secure scanning of single bits,which are usually complicated and expensive. Very unsharp timereferences may also be used for the implementation of the protocol,since the circuit 26 for carrying out the data transmission protocol canscan each single bit in a reliable manner in that it interprets thephase between two positive half waves as one bit each time. Depending onthe protocol used, the time references may even be entirely absent atthe stations, because the cycle duration of the AC voltage itself canalready serve as the time reference.

The AC voltage may also be a square-wave voltage or have some othershape, depending on the nature of the generator. It also possible forthe positive half wave to be longer than the negative half wave for thepurpose of a higher power transmission.

FIG. 4 shows the construction principle of the generator. The generatorcomprises an AC voltage source 41. The negative half wave is provided ata higher ohmic value than the positive half wave, so that the switches24 in the stations can be realized as low-power semiconductor switches.This is realized by means of series resistors 42, 43 which are connectedin parallel to diodes 44, 45 so that the positive half wave remainslow-ohmic. In this embodiment, one of the resistors with its paralleldiode may also be omitted, i.e. be replaced by a short-circuit, so thatone terminal of the generator 41 is directly connected to the conductor11 or 12. The generator may be an alternately switched H-bridge ofsemiconductor switches in the simplest case.

FIG. 5 shows the generator with the use of an additional transformer 46.The transformer is optional. The principle of the electrical supply andthe data transmission through two lines also works without transformer.The transformer, however, provides four further advantages. Firstly, thetwo half waves are rendered symmetrical and the interference radiationis thus minimized. Secondly, the primary voltage amplitude can bechanged into the voltage required for the application throughtransformation to the secondary voltage amplitude. Thirdly, the ACvoltage can be generated from a unipolar voltage source in a very simplemanner. Fourthly, the AC voltage may be supplied without reference tothe ground potential, so that a short-circuit of a conductor with someother potential, such as ground, or some other supply voltage isacceptable.

In a further embodiment of the system, the generator and station aresomewhat expanded so as to accommodate interruption defects. FIG. 6shows the expanded generator in which a transformer 47 with a centraltap is used, which tap is connected, for example, to the system ground48. The expanded station in FIG. 7 is provided with further diodes. Ifthere is an interruption defect in the line 11, the station can still besupplied from the positive half wave, albeit with half the secondaryvoltage only, via a current path from the system ground through diode51, capacitor 23 and loads connected in parallel therewith, diode 53,and conductor 12. Similarly, the amplitude of the negative half wave canstill be modified by means of switch 24 for the transmission of data,i.e. via the current paths of conductor 12, diode 56, switch 24, diode54, and system ground. The principle is the same for an interruption inconductor 12. The current path for the supply from the positive halfwave then runs through diodes 21 and 52. The current path for datatransmission runs through diodes 55 and 22.

These back-up modes of operation arise automatically without any activeswitch-over in the stations or in the generator. The transmissionremains a differential one for both half waves in the back-up modes,although usually the desired effect of a minimized interferenceradiation is no longer achieved because of the different properties ofthe conductive lines and system ground and the asymmetry of the voltagegradient in time in the two lines arising therefrom. In addition, thetolerance to short-circuit defects in lines 11 and 12 is limited insystems having this expanded construction for tolerating interruptiondefects, because the alternative current path via system groundconstitutes a grounding, and a short-circuit of one of the lines 11 and12 with, for example, ground will short-circuit the secondary winding.

This latter disadvantage may be eliminated as shown in FIG. 6 ashort-circuit protection of the generator, which has to be realizedanyway, is constructed, for example, as a precision fuse 49a and 49b, sothat a short-circuit defect activates the fuse and thus acts as aninterruption defect, which is already allowed for by the arrangementprovided. It is possible in this manner to tolerate either ashort-circuit defect or one or several interruption defects in oneconductor.

In an alternative embodiment of the system, the interruption defects areaccommodated in a somewhat different manner. The stations areconstructed in the same way as in FIG. 7, but the generator isconstructed without central tap at the transformer, as shown in FIG. 8,and has additional switches 57 and 58 which are controlled via thecontrol circuit 59. The control circuit 59 can close the two switchesalternately when an interruption defect has been detected, for examplein the case of a communication failure with a station, until thiscommunication is restored again. This embodiment requires positiveactions for tolerating the defect, unlike the embodiment describedabove. However, these actions are centrally controlled once and for all,the stations themselves need not take any action. This is still veryadvantageous compared with other, known defect-tolerant systems inwhich, for example, the actions are to be carried out at all stationsand also in synchronism with one another in accordance with ISO11992,until it has been ascertained in a series of communication trials whichof the lines are still available for use.

A further advantage of this embodiment is that at least short-circuitdefects to ground can be tolerated without any measures here because theground connection is not present when switches 57 and 58 are open. Theprecision fuses are accordingly absent in this embodiment. Short-circuitdefects or interruption defects can be accepted, the control circuit 59ascertaining, for example by voltage scanning, the absence of a possiblysimultaneously arising short-circuit before a switch 57 or 58 is closed.

A further advantage of this embodiment is that the full voltage sweep isavailable again to the stations in the case of an interruption defectafter ground potential has been applied to the respective, correctconductors at the generator side, whereas only half the peak value isavailable in the embodiment described above with a central tap at thetransformer. It is accordingly not necessary to overdimension thesupply--anticipating any defect condition--, which leads to a moreefficient conversion operation.

The disadvantage of an increased interference radiation in the case ofdefects remains in both embodiments which tolerate interruption defects.This can be eliminated through a small additional provision when, asshown in FIG. 9, a third conductor 13 is laid parallel to conductors 11and 12 instead of the system ground. The desired effect of a minimizedinterference radiation also in the back-up mode of operation is theresult again because the operation remains strictly differential. Theuse of the alternative conductor 13 instead of the system ground in theone embodiment of the system in addition provides a simultaneoustolerance of interruption defects and of a short-circuit of a conductorbecause the conductor 13 is also operated free from ground potential. Inaddition, a short-circuit between conductors is also tolerated, i.e.between the conductors 11 and 13 or the conductors 12 and 13, providedthe generator is protected against short-circuits. The advantage of thetolerance of a short-circuit between one conductor with either ground oranother supply potential and simultaneously of one or severalinterruption defects in one conductor is also obtained in the otherfurther embodiment of the system. Short-circuits between conductors 11and 13 or between 12 and 13 are tolerated in this case as well.

The minimal-cost embodiment of the invention shown in FIG. 10 is givenby way of contrast to the embodiment of the invention described abovewhich is highly tolerant to defects. The transmission of power and datacan be achieved with only a single conductor and an additionalconnection to the system ground in systems where an increasedinterference radiation is acceptable and a defect tolerance is notrequired, so that only one conductor, for example conductor 11, need beinstalled, which is very inexpensive. The other conductor, for exampleconductor 12, is replaced by the system ground. In this embodiment,moreover, a resistor and a diode can be dispensed with for thegenerator.

The invention can thus be optimally used for certain applications asregards the achievable cost reductions and the required tolerance todefects. If at least some of the stations should still remain capable ofoperation in those embodiments which are of a simpler circuitconstruction, for example in accordance with FIGS. 1 to 5 and 10,separation units can be inserted in at least one conductor,automatically disconnecting the short-circuited portion following theseparation unit, as seen from the generator, and automaticallyconnecting it again after the short-circuit has been eliminated.

FIG. 11 shows a system in which separation units for the disconnectionof entire system blocks (separation unit 17) are provided in the lines11 and 12 for the stations 14, 15 and 16, or in the supply line to astation for cutting off only this station (separation unit 18).Reference numeral 11a denotes that conductor portion which follows theseparation unit as seen from the generator 10 and which is disconnectedfrom the remainder of the system in case of a short-circuit or overload,together with all stations connected to this conductor portion. It isassumed in FIG. 11 that the lines 11 and 12 are connected in a ringarrangement and are returned again to the generator 10. Furtherseparation units (not shown) are present in this ring, such that atleast one station is present between two separation units each time. Ifa short-circuit arises in a conductor portion between two separationunits, only the stations present in this conductor portion will bedisconnected, whereas the rest of the system remains fully operational.

FIG. 12 is a block diagram representing a separation unit 17 which isconnected in the line 11. This separation unit 17 comprises a switch 60,a test resistor 63, and monitoring units 61 and 68, the monitoring unit68 being connected in series with the monitoring unit 61 and achievingthe connection between the control input 65 and the non-interrupted line12. The potential of the conductor portion 11a of the switch 60 istested at the output by means of the test line 62 of the monitoring unit61, and the potential of the conductor portion 11 is tested at the inputof the switch 60 by means of the test line 69 of the monitoring unit 68.The test resistor 63 is controlled via the control input 64 of themonitoring units 61 and 68.

FIG. 13 shows a circuit arrangement for realizing a separation unit inaccordance with FIG. 12. This arrangement comprises p-channel powertransistors 71, 72 and resistors 73, 74 as well as a capacitor 70 and adiode 75 in the switch 60, and an n-channel control transistor 80 and aresistor 81 in the monitoring unit 61. The capacitor 70 is arrangedbetween the junction point of the two sources of the power transistors71 and 72 of the switch 60 and the junction point of the interconnectedgates of the power transistors 71 and 72. The test resistor 63, actingas a current-limiting element which shunts the switch 60, is notcontrolled in this example and accordingly comprises merely the simpleohmic resistor 90.

Only a scanning of the line 11a at the output of the switch 60 iscarried out here with no more than one monitoring unit 61. A test line62 of the monitoring unit 61 for triggering of the control transistor 80via the resistor 81 scans the potential of the line 11a at the output ofthe switch 60. The switch 60 is included in the line 11, and the gatesof the two power transistors 71 and 72 are connected to the controlinput 65 via the voltage divider comprising the resistors 73 and 74. Acapacitor 70 is charged via the resistor 74 and the diode 75 only if apotential lower than the potential of the line 11 is applied to thecontrol input 65, and in that case the power transistors 71 and 72become conducting. During the negative half wave of the AC voltage, whenthe potential of the line 11 is lower than the potential of the line 12,the diode 75 prevents a reverse charging of the capacitor 70, so thatthe voltage required for closing the power transistors 71 and 72 ismaintained. The discharging of the capacitor 70 via the resistor 73 isto be so adjusted that no excessive change in the charge condition ofthe capacitor 70 occurs during the negative half wave.

The potential of the line 11 at the input of the switch 60 is conductedto the output of the switch 60 and to the line portion 11a via the testresistor 63 when the power transistors 71 and 72 are open. Thispotential is scanned by means of the test line 62 and supplied to thecontrol input of the monitoring unit 61 and therein via the resistor 81to the gate of the control transistor 80. When the potential of the testline 62 exceeds a first triggering threshold, the control transistor 80in the monitoring unit 61 is closed. The monitoring unit supplies thepotential of the line 12 to the control input 65 of the switch 60, sothat the voltage drop across the resistor 73 closes the powertransistors 71 and 72. The uninterrupted normal operation of theseparation unit is achieved in this manner.

If there is a short-circuit or an overload between the lines 11a and 12at the output of the separation unit, the potential conducted via thetest resistor 63 to the line portion 11a comes below the triggeringthreshold at the test input 62 of the monitoring unit 61 as required forclosing the control transistor 80, and the control transistor 80 of themonitoring unit 61 becomes non-conducting. As a result, the controlinput 65 cannot be connected to the potential of the line 12, and thevoltage necessary for closing the power transistors 71 and 72 cannot bebuilt up, so that the power transistors 71 and 72 are non-conducting andthe switch 60 remains open. The supply generator 10 (FIG. 1) and allstations not connected to this line portion 11a are separated from theshort-circuit thereby.

A test condition is created via the connection of the test resistor 63.When the short-circuit or the overload is eliminated, the potential canbuild up again across the test resistor and exceed the triggeringthreshold, so that the control transistors 80 and 80a connect thepotential of the line 12 through again, and the power transistors 71 and72 become conducting and the switch 60 is closed, whereby normaloperation is automatically reinstated.

The circuit arrangement of FIG. 13 recognizes direct short-circuits,short-circuits across resistors, and short-circuits with diodes whichare active in the positive half wave, but not short-circuits with diodeswhich are active in the negative half wave, i.e. with the anodeconnected to line 12 and the cathode to line 11. An embodiment for thiscase will be described further below.

FIG. 14 shows an embodiment of a separation unit wherein the switch 60is included in the line 12 which has the lower potential during thepositive half wave. In this example, again, only one scanning of theline 12a at the output of the switch 20 is shown. The test resistor 23is not controllable also in this embodiment, containing only a singleohmic resistor 50.

The n-channel power transistors 71 and 72 in the switch 60 areconducting only if a potential higher than the potential of the line 12is applied to the control input 65. The monitoring unit 61 comprises ap-channel control transistor 80 by which the potential of the line 11 ispassed on to the control input 65. This through connection is achievedin dependence on the potential of the line portion 12a which is scannedby the test line 62 at the output of the switch 60. During switching-on,the potential of the line 12 at the input of the switch 60 is conductedvia the test resistor 63 to the output of the switch 60 when the powertransistors 71 and 72 are non-conducting. This potential is scanned withthe test line 62 and conducted to the control input of the controltransistor 80 of the monitoring unit 61. The control transistor 80 isclosed when the potential passes above a triggering threshold. Thisconnects the control input 65 of the switch 60 to the potential of theline 11, which drives the power transistors 71 and 72 into theconducting state and initiates the normal operation of the separationunit. When a short-circuit or an overload occurs between the lines 11and 12, the potential at the line portion 12a does not reach the valueof the triggering threshold which is necessary for closing the controltransistor 80, and the control transistor 80 remains non-conducting. Thecontrol input 65 is thus no longer connected to the potential of theline 11, and the voltage necessary for closing the power transistors 71and 72 cannot be established. The generator 10 (FIG. 1) is separatedfrom the short-circuit in this manner. The test condition is achievedvia the test resistor 63. When a short-circuit occurs during operation,the potential at the line portion 12a sinks below the triggeringthreshold value, the control transistor 80 becomes non-conducting, andthe switch 60 interrupts the line 12. When the short-circuit between thelines 11 and 12a is eliminated again, the potential of the line portion12a at the output of the switch 60, and thus at the test line 62 of themonitoring unit 61, can again assume a value above the triggeringthreshold for closing the control transistor 80 of the monitoring unit61, and restore normal operation automatically.

The arrangement of FIG. 14, again, does not recognize short-circuitswith diodes which are active in the negative half wave of the ACvoltage.

The separation units shown in FIGS. 13 and 14 may also be used forrecognizing short-circuits occurring in the negative half wave of the ACvoltage in an analogous manner. In this case, the separation unit ofFIG. 13 must be included in the line 12, because the latter has thehigher potential during the negative half wave, and the separation unitof FIG. 14 must be included in the line 11, since the latter has thelower potential during the negative half wave. Short-circuits with adiode active in the positive half wave are not recognized in theseembodiments.

The examples of FIGS. 13 and 14 each provide only one scanning at theoutput of the switch 60. The circuits, however, may be complemented witha monitoring unit 68 for scanning also at the input of the switch 60 ofFIG. 12. The monitoring units should be connected in series in thatcase.

FIG. 15 shows an embodiment in which the potential of the line 11 ishigher than the potential of the line 12 during the positive half wave,while during the negative half wave the potential of the line 11 islower than the potential of the line 12. The circuit of FIG. 14 is thestarting point for the circuit of FIG. 15, in which also n-channeltransistors 71 and 72 are used in the switch 60. The switch 60 comprisesa capacitor 70 and a diode 75 for storing the charge which is necessaryfor the conducting state of the power transistors 71 and 72 during thepositive half wave. The control transistor 80 of the monitoring unit 61is a p-channel transistor, as in FIG. 14.

With the monitoring unit 61 without the elements 801 to 808,short-circuits in the negative half wave only are recognized, becausethe control transistor 80 must be triggered with a negative gate-sourcevoltage in order to be conducting. To render this monitoring unit 61suitable also for short-circuits which act only in the positive halfwave, the monitoring unit 61 is augmented with a circuit comprising theswitches 801, 803, and 804, the resistors 802, 806, and 807, thecapacitor 805, and the diode 808, for recognizing the short-circuitcondition via a diode whose anode is connected to the line 11 and whosecathode is connected to the line 12, during the positive half wave, soas to be able to control the switch 60 accordingly. When the powertransistors 71 and 72 are conducting, the capacitor 805 is charged to apositive value during the positive half wave via the current path:resistor 807→diode 808→resistor 806→diode in switch 804. The diode 808prevents a discharging during the negative half wave. The capacitor 805thus retains a voltage at which the switches 803 and 804 close.Discharging via the resistor 806 during the negative half wave is sodimensioned that the capacitor 805 can discharge to a low degree onlyduring a single cycle duration, while charging via the resistor 807 cantake place during a positive half wave. The continuously closed switches803 and 804 keep the switch 801 always open, so that the controltransistor 80 operates normally during the negative half wave in amanner analogous to that of the circuit of FIG. 14.

If the potential of the line 11 becomes lower than the potential of theline 12 during the negative half wave, the p-channel transistor 80 willreceive a negative gatesource voltage, so that it becomes conducting. Inthis condition, the capacitor 70 of the switch 60 can be charged via thecurrent path: switch 80→diode 75→resistor 74→resistor 73→diode in thepower transistors 72 and 71. A positive gate-source voltage thus arisesfor the power transistors 71 and 72, so that they both become conductingand the switch 60 is closed. Discharging of the capacitor 70 via theresistor 73 is so dimensioned that the potential across the capacitor 70changes only little over the duration of a cycle. The capacitor 70 ischarged via the resistor 74 in the duration of a negative half wave.

Short-circuits which lead to the same effect during both half waves aredirect (low-ohmic) short-circuits and short-circuits across a resistorbetween the lines 11 and 12, while the resistance value depends on thedimensioning of the separation unit. If a direct short-circuit or ashort-circuit across a resistor occurs at the output of the separationunit between the lines 11 and 12, the control transistor 80, which isnormally closed during the negative half wave, is opened. The controltransistor 80 is always open during the positive half wave. As a result,the control input 65 is not connected to the potential of the line 12 atany time any more, and the capacitor 70 is discharged via the resistor73. The positive gatesource voltage for the power transistors 71 and 72can thus not build up, so that the power transistors 71 and 72 becomenon-conducting. The generator 10 (FIG. 1) is thus separated from theshort-circuit, and the test condition with the connection via the testresistor 63 is established. The circuit portion with the switches 803,804, and 801 is immaterial to this situation with a directshort-circuit.

Short-circuits which occur only in the negative half wave, i.e. viadiodes whose anodes are connected to line 12 which has the higherpotential during the negative half wave and whose cathodes are connectedto line portion 11a which has the lower potential during the negativehalf wave, lead to the same situation as in the case described above,because the control transistor 80 remains non-conducting during thenegative half wave on account of the short-circuit and the controltransistor 80 is always non-conducting during the positive half wave.The capacitor 805 is not charged via the resistor 807 and the diode 808in the case of short-circuits which occur only in the positive halfwave, i.e. via diodes whose anodes are connected to the line 11 whichhas the higher potential during the positive half wave and whosecathodes are connected to the line 12 which has the lower potentialduring the positive half wave. The capacitor 805 is discharged via theresistor 806, and the switches 803 and 804 always remain open. Duringthe positive half wave, the control transistor 80 and the switch 801remain open at all times or are non-conducting. Upon the transition tothe negative half wave, the switch 801 is closed, so that the controltransistor 80 is given a gatesource voltage below its switching-onthreshold and thus remains non-conducting also during the negative halfwave. The control input 65 is thus no longer connected to the potentialof the line 12 at any time, and the capacitor 70 is discharged via theresistor 73. The power transistors 71 and 72 are non-conducting, and theswitch 60 is open. The generator 10 (FIG. 1) is thus separated from theshort-circuit and the test condition is obtained by means of the soleconnection via the test resistor 63. If a short-circuit occurs duringoperation, the monitoring unit 61 detects short-circuits between thelines 11a and 12 in the manner described above, and the powertransistors 71 and 72 of the switch 60 are quickly opened. When theshort-circuit between the lines 11a and 12 is eliminated again, thepotential at the output of the switch 60 can again assume a value viathe test resistor 63, and thus at the test line 62 of the monitoringunit 61, at which the control transistor 80 is conducting and normaloperation is automatically reinstated.

The arrangement of the switch 60 in the line 11, which has a lowerpotential than the line 12 only during the negative half wave, meansthat all short-circuits occurring during the negative half wave aredetected with the control transistor 80 of the monitoring unit 61. Theadditional circuit comprising the switches 804, 803, and 801 in themonitoring unit 61 in addition detects those short-circuits which occurin the positive half wave.

If in contrast to the arrangement shown in FIG. 15 the switch 60comprising n-channel transistors is arranged in the line 12, which hasthe lower potential only during the positive half wave, while themonitoring unit 61 represents the connection to the line 11 via thep-channel control transistor 80, then the control transistor 80 of themonitoring unit 61 detects all short-circuits which occur during thepositive half wave. The additional circuit comprising the switches 804,803, and 801 in the monitoring unit 61 recognizes the furthershort-circuits which may occur in the negative half wave.

Another, similar embodiment may be indicated on the basis of theprinciple of the circuit arrangement of FIG. 13. The switch 60 comprisesp-channel transistors in line 11, and the control transistor 80 is ann-channel transistor. With the switch 60 in the line 11, which has thehigher potential only during the positive half wave, all short-circuitsoccurring during the positive half wave are now detected by means of thecontrol transistor 80 of the monitoring unit 61 which forms theconnection to line 12. The additional circuit comprising the switches804, 803, and 801 in the monitoring unit 61 detects the furthershort-circuits which may arise in the negative half wave. The switches71, 72, 704, and 803 are then constructed as p-channel transistors, andthe switches 80 and 801 as n-channel transistors.

If the switch 60 comprising two p-channel transistors is included inline 12, which has the higher potential only during the negative halfwave, all short-circuits occurring during the negative half wave aredetected with the control transistor 80 of the monitoring unit 61 whichforms the connection to the line 11. The additional circuit comprisingthe switches 804, 803, and 801 in the monitoring unit 61 then detectsthe further short-circuits which may arise in the positive half wave.The switches 71, 72, 804, and 803 re then constructed as n-channeltransistors, and the switches 80 and 801 as p-channel transistors.

The embodiments just mentioned faultlessly recognize all three kinds ofshort-circuits mentioned, while in the case of a short-circuit via aresistor the value of the resistance which is still regarded as ashort-circuit depends on the value of the test resistor 90 and thecurrent which flows during normal operation.

A further possibility for detecting short-circuits at an AC voltage andfor controlling a power transistor is to provide one separation unit forthe negative half wave and one for the positive half wave. For thepositive half wave, when the potential of the line 11 is higher than thepotential of the line 12, a switch 60 in accordance with FIG. 13 isprovided in the line 11, or in accordance with FIG. 14 in the line 12.For the negative half wave, when the potential of the line 11 is lowerthan the potential of the line 12, a switch 60 in accordance with FIG.13 is provided in the line 12, or one in accordance with FIG. 14 in theline 11. Both separation units must be functionally connected in series,so that data and power of the generator 10 can only be connected throughwhen both separation units have detected no short-circuit. Thisarrangement requires a larger number of power transistors, this incontrast to the arrangement of FIG. 15.

FIG. 16 shows an embodiment in which the switch 60 is included in theline 12 in accordance with the basic arrangement of FIG. 14. Themonitoring unit 61 now detects with the control transistor 80 allshort-circuits which occur during the positive half wave, and with theswitches 801, 803, and 804 the further short-circuits which occur duringthe negative half wave. To increase the detection reliability forshort-circuits during the positive half wave, a hysteresis is providedfor the triggering threshold of the control transistor 80. A hightriggering threshold obtains during normal operation of the separationunit with the switch 60 closed so as to be able to open the switch 60quickly, with the purpose of avoiding an overload on the generator 10(FIG. 1), when the voltage at the output of the separation unit drops,which voltage is scanned with the test line 62 via the resistor 81during the positive half wave. When the switch 60 is open, however, alow triggering threshold obtains so that a faultless testing forshort-circuits during the positive half wave can be achieved via thehigh-ohmic test resistor 63, and the normal load of other stations inthe system cannot simulate a short-circuit.

This hysteresis is achieved by means of the switches 813 and 814 and thezener diode 811. When the control transistor 80 is conducting during thepositive half wave, the switch 60 is also closed. This information isutilized for keeping the switch 814 closed in that the capacitor 817charges itself via the control transistor 80, the diode 819, and theresistor 818 during the positive half wave. This charge condition ismaintained during the negative half wave, because the discharging viathe resistor 816 is set so low that the charge condition hardly changes.The switch 813 is kept open by the closed switch 814, and the zenerdiode 811 thus determines the triggering threshold for switching of thecontrol transistor 80. The adjustment of the triggering threshold mustsafeguard then that the control transistor 80 is closed so long duringthe positive half wave that the capacitors 70 and 817 in the switch 60and in the hysteresis definition circuit of the monitoring circuit 61can become sufficiently charged. If the control transistor 80 isnon-conducting during the positive half wave, the switch 60 is alsoopen, and there will only be a connection between the input and theoutput of the switch 60 via the test resistor 63. This information isutilized for adjusting the triggering threshold for the controltransistor 80 during the positive half wave to a low voltage value inthat the capacitor 817 is discharged via the resistor 816, and theswitch 814 is opened. This closes the switch 813 during the positivehalf wave, so that the zener diode 811 is shunted.

To improve the disconnection properties in the transmission of powerand/or data by the switch 60, the test resistor 63 must not assume a toolow ohmic value. A conflicting requirement is that a minimum voltagelevel must be present at the output of the switch so as to recognizereliably the normal situation, i.e. no short-circuit. In the case oflarge loads, i.e. a low load resistance behind the switch, the testresistor must be chosen to be correspondingly low-ohmic. In the systemdescribed, which transmits power during the positive half wave andaccordingly has a low-ohmic load, and which transmits data during thenegative half wave and is accordingly given a higher-ohmic load, thereis the possibility to adjust the value of the test resistor 63 independence on the situation. For this purpose, the status of the line12a at the output of the switch is scanned during the negative half wavewith the switch 82 of the monitoring unit 61 via the resistor 43 and thetest line 62. If the negative half wave is capable of building upnormally, the switch 82 is closed, and the shunting capacitor 904 in thecontrol unit 91 of the test resistor 63 is charged via the control line64. The charge condition is retained during the positive half wave andthus keeps the switch comprising the transistors 906 and 907 in thecontrol unit 91 continually closed, so that the partial resistor 903 ofthe resistor unit 90 is shunted. This renders the portion of low ohmicvalue of the test resistor 63, comprising the partial resistors 901 and902, active between the input and the output of the separation unit. Ifthe negative wave is incapable of building up, i.e. in the case of ashort-circuit between the lines 11 and 12a, the switch 82 is not closed,and then the switch comprising the transistors 906 and 907 in thecontrol unit 91 of the test resistor 63 cannot close either, so that thepartial resistor 903 is not shunted. This renders the sum of the partialresistance values 901, 902, and 903 of the test resistor 63 activebetween the input and the output of the switch 60.

A two-stage switching-on of the separation unit is achieved by means ofthis arrangement, i.e. a short-circuit test is first made during thenegative half wave with the high-ohmic test resistor 63. Directshort-circuits, short-circuits via a low-ohmic resistor, andshort-circuits via a diode whose anode is connected to line 12 and whosecathode is connected to line 11 are recognized thereby. Only when suchshort-circuits are not present, a switch is made to a test resistor oflower ohmic value, which renders it possible to detect a short-circuitvia a diode whose anode is connected to line 1 and whose cathode isconnected to line 12, during the positive half wave. When the switch 60is open, there is a high-ohmic connection between the input and theoutput of the switch 60 during the positive half wave via the testresistor 63, i.e. a satisfactory decoupling of the still operationalsystem from those system portions which are hit by the short-circuit.The sum of the partial resistance values 901, 902, and 903 of the testresistor 63 is so dimensioned that a sufficiently strong signal arisesat the output of the switch 60 for the high-ohmic negative half wave, sothat normal operation and any short-circuit which may be present can bereliably distinguished. Similarly, the sum of the constituent resistancevalues 901 and 902 must be such that normal operation and ashort-circuit during the positive half wave can be reliablydistinguished.

In these embodiments, n-channel and p-channel transistors were used inthe monitoring units 61 and 68 for detecting a short-circuit and forswitching the control input of the switch 60. The use of suchtransistors constitutes no more than one possible embodiment by means ofwhich a short-circuit can be recognized in a simple and reliable manner.Other possibilities are formed by the use of, for example, voltagecomparators and similar components. The switches may be alternativelyconstructed with bipolar transistors.

What is claimed is:
 1. A system comprising at least two stations whichare interconnected by means of at least two conductors for thetransmission of data and power, and comprising a power source coupled tosaid conductors, characterized in that the power source is designed fordelivering a bipolar AC voltage having pulses of a first and a secondpolarity to the conductors, in that means for absorbing and storingpower exclusively from the pulses having the first polarity are providedin each station without its own power supply and are connected to theconductors, in that modifying means are connected to the conductors ineach of the stations provided for the transmission of data, which meansmodify exclusively pulses of the AC voltage having the second polarityin dependence on the data to be transmitted, and in that a detector forevaluating the amplitude of those pulses only which have the otherpolarity is connected to the conductors in each of the stations providedfor the reception of data.
 2. A system as claimed in claim 1,characterized in that the power source is designed for delivering thepulses of the AC voltage having the first polarity to the conductors ata lower ohmic value than the pulses having the other polarity.
 3. Asystem as claimed in claim 2, characterized in that the power source isconnected to the conductors via at least one resistor with a rectifierconnected in parallel thereto.
 4. A system as claimed in claim 1,characterized in that the power source comprises an AC voltage generatorand a transformer having a primary winding and a secondary winding,which primary winding is coupled to the generator and which secondarywinding is coupled to the conductors.
 5. A system as claimed in claim 4,characterized in that a third conductor is provided which is coupled tothe power source and to the stations.
 6. A system as claimed in claim 5,characterized in that the secondary winding comprises a centralconnection which is coupled to the third conductor.
 7. A system asclaimed in claim 5, characterized in that two switches and a controlunit are provided, preferably in the power source, which switches arearranged between the first and the third conductor and between thesecond and the third conductor, respectively, while the control unitcloses the relevant switch in the case of an interruption in the firstor the second conductor.
 8. A system as claimed in claim 2,characterized in that one of the conductors is formed by a groundconnection.
 9. A system as claimed in claim 1, characterized in that aseparation unit is included in at least one conductor, each separationunit subdividing the relevant conductor into a first and a secondconductor portion and comprising a switch connected to the conductorportions and a monitoring unit which is connected to at least oneconductor portion and is constructed for operating the switch when avoltage at at least one of the connected conductor portions rises toabove a first triggering threshold or drops to below a second triggeringthreshold, and in that the switch is shunted by a current-limitingelement.
 10. A station for use in a system as claimed in claim 1 withconnection terminals for at least two conductors for receiving power foroperating circuits in the station and for receiving and/or transmittingdata, wherein the following are coupled to the conductors:voltage supplymeans for deriving from the conductors power exclusively from the pulseshaving the first polarity and for storing this power, modifying meansfor modifying exclusively pulses having the second polarity on theconductors in dependence on data to be transmitted, a detector forevaluating the amplitude of the second polarity only.
 11. A station asclaimed in claim 10, characterized in that the means comprise arectifying arrangement and an energy storage device, which rectifyingarrangement supplies electric power to the energy storage deviceexclusively during the pulses of the first polarity, and the energystorage device is coupled to voltage supply terminals of electroniccircuits in the station.
 12. A station as claimed in claim 10,characterized in that the voltage supply means comprise a seriesarrangement of a first rectifier, the energy storage device, and asecond rectifier, which arrangement is connected between the first andthe second conductor, and in that two connection terminals of the energystorage device are connected to a third conductor, which is coupled tothe power source and to the stations via further rectifiers.
 13. Astation as claimed in claim 10, characterized in that the modifyingmeans comprise a series arrangement of a first rectifier, a switchableimpedance, and a second rectifier, and in that the junction pointsbetween the impedance and the first and the second rectifier are eachconnected to a third conductor, which is coupled to the power source andto the stations via respective further rectifiers.
 14. A separation unitfor a system as claimed in claim 9, characterized in that the switchcontrolled by the monitoring unit connects the conductor subdivided bythis switch when the voltage exceeds the first triggering threshold andinterrupts this conductor when the voltage drops to below the secondtriggering threshold.
 15. A separation unit as claimed in claim 14,characterized in that the monitoring unit is connected to both conductorportions.
 16. A separation unit as claimed in claim 14, characterized inthat the switch is constructed for the conduction of a current in bothdirections.
 17. A separation unit as claimed in claim 14, characterizedin that the switch comprises a first and a second field effecttransistor of the same conductivity type which are connected in seriesbetween the two conductor portions and whose gates are interconnectedand are coupled to the monitoring unit.
 18. A separation unit as claimedin claim 17, characterized in that the gates of the first and the secondfield effect transistor are connected to the junction point of the firstand the second field effect transistor via a capacitor and a dischargingelement connected in parallel thereto and are connected to a third fieldeffect transistor via a diode.
 19. A separation unit as claimed in claim18, characterized in that the capacitor in the switch stores the chargenecessary for closing the first and the second field effect transistorin the half wave of the AC voltage which is opposed to the triggeringthreshold.
 20. A separation unit as claimed in claim 17, characterizedin that the monitoring unit comprises a third field effect transistorfor each connected conductor portion, which transistor is connectedbetween the non-subdivided conductor and the gates of the first and thesecond field effect transistor, and whose gate is coupled to theconnected conductor portion, such that a connection of the monitoringunit to both conductor portions leads to a series connection of theassociated third field effect transistors.
 21. A separation unit asclaimed in any one of the claims 19 and 20, characterized in that theconductivity type of the first and the second field effect transistor isopposed to the conductivity type of the third field effect transistors,while the conductivity type of the field effect transistors determinesin which half wave of the AC voltage the triggering threshold for themonitoring unit lies.
 22. A separation unit as claimed in claim 19,characterized in that the monitoring unit is provided with additionalcircuit portions which render the third field effect transistornon-conducting in the case of an overload in the half wave not monitoredby means of the triggering thresholds.
 23. A separation unit as claimedin claim 14, characterized in that the triggering threshold of themonitoring unit has a hysteresis, while the triggering threshold is highin the case of closed switches and the triggering threshold is low inthe case of opened switches.
 24. A separation unit as claimed in claim14, characterized in that the current-limiting element comprises testresistors which can be switched over between a high-ohmic state and alow-ohmic state.
 25. A separation unit as claimed in claim 24,characterized in that the high-ohmic test resistor is activated when theswitch in a conductor is open, and the test resistor has a lower ohmicvalue owing to shunting of partial resistors when the switch is closed.